Switching power supplies are popular for high power applications because of the high efficiency and small area/volume required. Buck converters in particular are well suited to providing the high current at low voltages needed by high performance digital integrated circuits such as microprocessors, graphics processors, and network processors. For example, a buck converter is often used to step down a DC voltage (typically referred to as the input voltage) to a lower DC voltage (typically referred to as the output voltage). Since the power stage is fully switched (i.e., the power MOSFET is fully off or on), there is very little loss in the power stage and the converter power efficiency is very high.
Many power supplies operate in conjunction with current sensors to monitor the current in the power supply and the load. Referring to FIG. 4, one widely used lossless current sensing technique is known as inductor DCR current sense. For simplification, FIG. 4 illustrates the widely used “output reference” equivalent form, where the output voltage is shown as ground, and the voltage applied across the inductor (Vin-Vout) is shown as V1. By adding the RC network in parallel with the inductor L with its parasitic DC resistance r, and matching the time constants, the voltage across the capacitor C is proportional to the current through the inductor multiplied by the DCR. A voltage amplifier can then be used by the PWM controller to generate the desired signal representing the current through the inductor. This method is popular because the DCR of inductors is well controlled and characterized for tolerance and temperature variation, resulting in accurate current sensing.
This lossless current sense method, however, does not present a constant DC impedance to the sense amplifier. Instead, the choice of R is dependent on C, DCR and L. A further limiting factor is that the value choices for C are restricted to commonly widely spaced commercially available values, such as 0.1 uf, 0.12 uf, 0.15 uf, 0.18 uf, 0.22 uf, 0.27 uf and 0.33 uf. Consequently, a constant choice for R cannot be practically selected to match an arbitrary choice of L and DCR. Further, the gain of the sensed voltage presented to the sense amplifier is dependent on the DCR of the inductor.
FIG. 5 shows such a lossless inductor current sensing system applied to a DC/DC buck converter topology. A current sense amplifier (A1) associated with the buck controller interfaces between the current sense network and PWM control circuitry. The current sense amplifier topology may or may not be sensitive to the impedance of the sense network. The DC impedance of the network as presented to the sense amplifier is a function of the L and DCR values and therefore varies for different inductors. Further, the gain of the sense network is not a constant, but is a function of the DCR value. As a result, the dynamic range requirements of the sense amplifier are a function of DCR and the gain of signal representing the inductor current and may vary from one application to the next.